U.S. patent application number 10/093799 was filed with the patent office on 2002-12-12 for magnetic resonance apparatus.
This patent application is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Yasuda, To-Oru.
Application Number | 20020186012 10/093799 |
Document ID | / |
Family ID | 19012662 |
Filed Date | 2002-12-12 |
United States Patent
Application |
20020186012 |
Kind Code |
A1 |
Yasuda, To-Oru |
December 12, 2002 |
Magnetic resonance apparatus
Abstract
Disclosed is a magnetic resonance apparatus capable of measuring
a distribution of impurities in a sample at high resolution without
destroying the sample. A magnetic resonance apparatus has a
cryostat for cooling and holding a sample, coils for applying an
inclined magnetic field to a sample so that the intensity of the
magnetic field has a gradient to the reference axis of the sample,
a driving mechanism for moving the sample to which the magnetic
field is applied with respect to the magnetic field, antennas for
irradiating the sample to which the magnetic field is applied with
an electromagnetic wave; and antennas for detecting an
electromagnetic wave emitted from the sample irradiated with the
electromagnetic wave.
Inventors: |
Yasuda, To-Oru; (Hyogo,
JP) |
Correspondence
Address: |
McDERMOTT, WILL & EMERY
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha
|
Family ID: |
19012662 |
Appl. No.: |
10/093799 |
Filed: |
March 11, 2002 |
Current U.S.
Class: |
324/318 ;
324/309 |
Current CPC
Class: |
G01R 33/44 20130101;
G01N 24/085 20130101 |
Class at
Publication: |
324/318 ;
324/309 |
International
Class: |
G01V 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 6, 2001 |
JP |
2001-170801(P) |
Claims
What is claimed is:
1. A magnetic resonance apparatus comprising: holding means for
cooling and holding a sample containing atomic nuclei having a
magnetic moment; magnetic field applying means for applying a
magnetic field which is inclined so that the magnetic field
intensity has a gradient to a reference axis of the sample; driving
means for driving the sample to which the magnetic field is applied
with respect to the magnetic field; irradiating means for
irradiating the sample to which the magnetic field is applied with
an electromagnetic wave; and detecting means for detecting an
electromagnetic wave emitted from the sample irradiated with the
electromagnetic wave.
2. The magnetic resonance apparatus according to claim 1, wherein
said magnetic field applying means Zeeman-splits the atomic nuclei
by applying the magnetic field to the sample.
3. The magnetic resonance apparatus according to claim 1, wherein
said irradiating means and said detecting means include common
antenna devices.
4. The magnetic resonance apparatus according to claim 1, wherein
said irradiating means includes a maser.
5. The magnetic resonance apparatus according to claim 1, further
comprising control means for controlling an operation of said
driving means.
6. The magnetic resonance apparatus according to claim 1, wherein
said driving means can move the sample along three axes which are
linearly independent of each other.
7. The magnetic resonance apparatus according to claim 1, further
comprising a diamagnetic member provided near said magnetic field
applying means so as to increase the magnetic field intensity near
the sample.
8. The magnetic resonance apparatus according to claim 1, wherein
said sample is an impurity-doped semiconductor, and said magnetic
resonance apparatus measures the concentration of an impurity in
said semiconductor in accordance with the electromagnetic wave
detected by said detecting means.
9. A magnetic resonance apparatus comprising: holding means for
cooling and holding a sample containing atomic nuclei having a
magnetic moment; magnetic field applying means for sweeping and
applying a magnetic field which is inclined so that the magnetic
field intensity has a gradient to a reference axis of the sample;
irradiating means for irradiating the sample to which the magnetic
field is applied with an electromagnetic wave; and detecting means
for detecting the electromagnetic wave emitted from the sample
irradiated with the electromagnetic wave.
10. The magnetic resonance apparatus according to claim 9, wherein
said magnetic field applying means Zeeman-splits the atomic nuclei
by applying the magnetic field to the sample.
11. The magnetic resonance apparatus according to claim 9, wherein
said irradiating means and said detecting means include common
antenna devices.
12. The magnetic resonance apparatus according to claim 9, wherein
said irradiating means includes a maser.
13. The magnetic resonance apparatus according to claim 9, further
comprising control means for controlling the intensity of the
magnetic field applied to the sample by said magnetic field
applying means.
14. The magnetic resonance apparatus according to claim 9, wherein
said magnetic field applying means includes a pair of first coils
provided along a first axis, a pair of second coils provided along
a second axis linearly independent of said first axis, and a pair
of third coils provided along a third axis linearly independent of
said first and second axes.
15. The magnetic resonance apparatus according to claim 9, further
comprising a diamagnetic member provided near said magnetic field
applying means so as to increase the magnetic field intensity near
the sample.
16. The magnetic resonance apparatus according to claim 9, wherein
said sample is an impurity-doped semiconductor, and said magnetic
resonance apparatus measures the concentration of an impurity in
said semiconductor in accordance with the electromagnetic wave
detected by said detecting means.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a magnetic resonance
apparatus and, more particularly, a magnetic resonance apparatus
for measuring a concentration distribution of impurities
distributed in a semiconductor.
[0003] 2. Description of the Background Art
[0004] In the case of forming a transistor on a silicon substrate
as a semiconductor, an impurity such as phosphorus, arsenic, boron,
or the like is ionized and implanted into a predetermined area in a
substrate at high speed and, after that, annealing is carried out
to recover a damage caused at the time of the implantation. A
three-dimensional concentration distribution of the impurity exerts
a large influence on the performances of a transistor. It is
therefore very important to know the distribution of an impurity in
an actual device in order to fabricate a semiconductor device of
high quality.
[0005] The following methods are known as methods of obtaining an
impurity concentration distribution.
[0006] (1) Simulation
[0007] Usually, an impurity concentration distribution in a
semiconductor device is measured by performing a simulation on the
basis of parameters of a fabricating process and an actually
measured physical quantity. The parameters in the fabricating
process of a semiconductor device and a procedure of obtaining an
impurity concentration distribution from the parameters by running
a simulation will be described hereinbelow.
[0008] Methods of introducing an impurity into a semiconductor
device are roughly divided into implantation and diffusion.
[0009] In the case of implanting an impurity, discharge takes place
in a source gas containing impurity atoms. Only target ions are
separated from generated ions by a mass spectrograph. The target
ions are accelerated by an electric field and implanted in a
semiconductor substrate. At this time, the speed of the ions to be
implanted is known from the intensity of the electric field. By
monitoring a current value of an ion beam, the number of ions
implanted can be known. Further, from an angle formed between the
ion beam and the semiconductor substrate, the direction of entry of
the impurity into the semiconductor substrate can be known.
[0010] By preliminarily forming a photoresist or the like as a mask
in an area to which an impurity is not desired to be implanted on
the surface of the semiconductor substrate so that the entry of
ions can be checked, the ions can be implanted only to a desired
area.
[0011] In order to obtain the concentration distribution of an
impurity in a semiconductor device in accordance with the
implantation parameters as described above, it is sufficient to
know the ion speed dependency of implantation depth of ions in a
silicon of the semiconductor substrate (what is called nucleus
stopping power). The value varies according to nuclear species and
has to be obtained by experiments. For this purpose, the depths
from the surface of ions implanted at various speeds have to be
analyzed by secondary ion mass spectrometry or the like.
[0012] In the case of doping an impurity by a diffusion method, a
semiconductor substrate is heated in an atmosphere containing the
impurity. According to the method, an impurity is introduced from
the surface of the semiconductor substrate where the concentration
is high to the inside by using a phenomenon that a substance moves
from an area of high concentration to an area of low concentration.
The degree of thermal diffusion can be controlled by the
temperature of the semiconductor substrate and time of maintaining
the temperature. In order to obtain the concentration distribution
of an impurity in a device from such diffusion parameters, a
diffusion coefficient of the impurity in silicon has to be
obtained. The diffusion coefficient is experimentally obtained.
[0013] As described above, when the physical quantity (the nucleus
stopping power or diffusion coefficient in the above case) related
to material properties in silicon of the impurity is calculated
once by any method, a three-dimensional concentration distribution
of the impurity in silicon can be obtained by a simulation from the
parameters in the fabricating process.
[0014] (2) Secondary Ion Mass Spectroscopy (SIMS)
[0015] In the case of actually recognizing a distribution of
impurity concentration in a semiconductor substrate, secondary ion
mass spectroscopy (SIMS) is used. In the SIMS, the surface of the
semiconductor substrate is irradiated with primary ions of Ar.sup.+
or the like at an energy of 100 eV to 30 keV, and charge particles
(secondary ions) out of particles emitted from the surface of the
silicon substrate by a sputtering phenomenon are analyzed by mass
spectroscopy, thereby identifying the kind and quantity of an
element. By scanning the surface with a primary ion beam, a
two-dimensional impurity distribution can be obtained.
[0016] Atoms exposed at the observed surface are removed by
observation and the observation is made while always exposing a new
surface, so that information of the impurity concentration in the
depth direction can be also obtained.
[0017] The resolution in the two-dimensional direction of the SIMS
depends on a converging system of the primary ion beam. Although a
Ga ion beam which is excellently converged can be converged to a
spot having a diameter of about 20 nm, the number of ions sputtered
decreases by narrowing the diameter of the primary ion beam, so
that the sensitivity deteriorates.
[0018] The lower the energy of the primary ion is, the higher the
resolution in the depth direction is, and the highest resolution is
about 1 nm. It is however difficult to obtain a low-energy ion beam
at a sufficient current density, so that the strength of the
secondary ion deteriorates.
[0019] Since a matrix effect exerts a large influence on sputtering
with low energy, it is difficult to perform a quantitative
analysis. In a practical application having sufficient detection
sensitivity, therefore, the spatial resolution is about hundreds
nm. Consequently, an apparatus used for the application may be
called an IMA (Ion Microprobe Analyzer). There is also an apparatus
in which an electron microscope is assembled to improve accuracy of
position information.
[0020] (3) Electron Microscope
[0021] Electron microscopes are roughly divided into two types, a
scanning type and a transmission type.
[0022] A scanning electron microscope (SEM) scans a sample
two-dimensionally with an electron beam converged narrowly,
captures secondary electrons emitted from the surface of the
sample, and re-constructs the shape of the sample surface from the
intensity distribution of the secondary electrons.
[0023] Since the emission of secondary electrons varies according
to the surface of a material, the differences in materials appear
as a contrast in an image. The resolution of an image is as high as
a few nm, and the focal depth is deep, so that it is advantageous
to evaluate the shape of a rough sample.
[0024] A transmission electron microscopy (TEM) irradiates a sample
of a thin film (having a thickness of a few nm to tens nm) with a
high-speed electron beam, captures the electron beam passed through
the sample, and observes an atomic arrangement or a lattice defect
of a sample. By using an electron beam having an energy of about
200 keV, resolution of about 0.2 nm is attained.
[0025] As application technologies of the electron microscopes,
various methods such as a scanning Auger electron microscope, an
optical microscope using an X-ray as a source of excitation, an
electron probe microanalysis for detecting an X-ray emitted, and a
combination of each of the techniques with a magnetic resonance
phenomenon are being developed and proposed.
[0026] (4) Magnetic Resonance Imaging (MRI)
[0027] According to the MRI, the density of atomic nuclei to be
observed in a matrix can be observed in a non-destructive manner.
By the conventional MRI, the atomic nuclei of hydrogen are objects
to be detected and a distribution of water components of an organic
sample such as a human body is observed. Since a human body or the
like is an object to be detected, an apparatus of MRI is
constructed so as to deal with a sample of a certain size.
[0028] In the method of obtaining the impurity concentration
distribution by a simulation, in reality, the implantation and
diffusion phenomenon of the impurity may vary according to the
states of the semiconductor substrate of the degree of a crystal
defect in a semiconductor substrate, an internal stress, and the
like. Consequently, there is a problem such that the impurity
concentration distribution obtained by a simulation is not always
correct.
[0029] According to the method by the SIMS, a sample is destroyed
by observation. There is consequently a problem such that a sample
cannot be observed in a non-destructive manner.
[0030] Further, out of electron microscopes, by the SEM, only the
information of the surface can be obtained and there is a problem
such that the kind of the element cannot be identified.
[0031] Further, the TEM has a problem such that only a sample
through which an electron beam can pass can be measured.
[0032] The MRI has a low spatial resolution which is at most about
10 .mu.m.
SUMMARY OF THE INVENTION
[0033] The invention has been achieved to solve the problems as
described above, and an object of the invention is to provide a
magnetic resonance apparatus capable of measuring an impurity
concentration distribution in all the area of a sample at high
resolution without destroying the sample.
[0034] A magnetic resonance apparatus according to an aspect of the
invention has: holding means for cooling and holding a sample
containing atomic nuclei having a magnetic moment; magnetic field
applying means for applying a magnetic field which is inclined so
that the magnetic field intensity has a gradient to a reference
axis of the sample; driving means for driving the sample to which
the magnetic field is applied with respect to the magnetic field;
irradiating means for irradiating the sample to which the magnetic
field is applied with an electromagnetic wave; and detecting means
for detecting an electromagnetic wave emitted from the sample
irradiated with the electromagnetic wave.
[0035] In the magnetic resonance apparatus constructed as described
above, a sample is cooled and held by the holding means. In a state
where a magnetic field is applied to the sample, an electromagnetic
wave is emitted from the irradiating means, so that the intensity
of the electromagnetic wave emitted increases. As a result,
sufficient signal intensity for improving the resolution can be
obtained, and the resolution of the magnetic resonance apparatus
can be improved. The distribution of atomic nuclei in the sample
can be also known without destroying the sample. Further, the
driving means can move the sample with respect to the magnetic
field, so that the inclined magnetic field is applied to the entire
sample. Thus, the distribution of atomic nuclei of not only the
surface but also the inside of the sample can be known.
[0036] Preferably, the magnetic field applying means Zeeman-splits
the atomic nuclei by applying the magnetic field to the sample.
[0037] Preferably, the irradiating means and the detecting means
include common antenna devices. In this case, the antenna devices
can function as both irradiating means and detecting means, so that
the number of components can be reduced.
[0038] Preferably, the irradiating means includes a maser. In this
case, since the maser has generally a narrow bandwidth and
generates a high output, the resolution can be improved by using
the maser.
[0039] Preferably, the magnetic resonance apparatus further
includes control means for controlling an operation of the driving
means.
[0040] Preferably, the driving means can move the sample along
three axes which are linearly independent of each other. In this
case, the driving means can move the sample along the three axes
which are linearly independent of each other, so that the
distribution of impurity concentration in the sample on the three
axes which are linearly independent of each other can be known. As
a result, the three-dimensional impurity concentration distribution
in the sample can be known.
[0041] Preferably, the magnetic resonance apparatus further
includes a diamagnetic member provided near the magnetic field
applying means so as to increase the magnetic field intensity near
the sample. In this case, since the diamagnetic member increases
the magnetic field intensity near the sample, the resolution can be
further improved.
[0042] Preferably, the sample is an impurity-doped semiconductor,
and the magnetic resonance apparatus measures the concentration of
an impurity in the semiconductor in accordance with the
electromagnetic wave detected by the detecting means.
[0043] A magnetic resonance apparatus according to another aspect
of the invention has: holding means for cooling and holding a
sample containing atomic nuclei having a magnetic moment; magnetic
field applying means for sweeping and applying a magnetic field
which is inclined so that the magnetic field intensity has a
gradient to a reference axis of the sample; irradiating means for
irradiating the sample to which the magnetic field is applied with
an electromagnetic wave; and detecting means for detecting the
electromagnetic wave emitted from the sample irradiated with the
electromagnetic wave.
[0044] In the magnetic resonance apparatus constructed as described
above, a sample is cooled and held by the holding means. In a state
where a magnetic field is applied to the sample, an electromagnetic
wave is emitted from the irradiating means, so that the intensity
of the electromagnetic wave emitted increases. As a result,
sufficient signal intensity for improving the resolution can be
obtained, and the resolution of the magnetic resonance apparatus
can be improved. The distribution of atomic nuclei in the sample
can be also known without destroying the sample. Further, since the
gradient magnetic field is swept and applied, the gradient magnetic
field is applied to the entire sample. Thus, the distribution of
atomic nuclei of not only the surface but also the inside of the
sample can be known.
[0045] Preferably, the magnetic field applying means Zeeman-splits
the atomic nuclei by applying the magnetic field to the sample.
[0046] Preferably, the irradiating means and the detecting means
include common antenna devices. In this case, the antenna devices
function as both the irradiating means and the detecting means, so
that the number of components can be reduced.
[0047] Preferably, the irradiating means includes a maser. In this
case, since the maser has generally a narrow bandwidth and
generates a high output, the resolution can be improved by using
the maser.
[0048] Preferably, the magnetic resonance apparatus further
includes control means for controlling the intensity of the
magnetic field applied to the sample by the magnetic field applying
means.
[0049] Preferably, the magnetic field applying means includes a
pair of first coils provided along a first axis, a pair of second
coils provided along a second axis linearly independent of the
first axis, and a pair of third coils provided along a third axis
linearly independent of the first and second axes. In this case,
the driving means can sweep and apply the magnetic field along the
three axes which are linearly independent of each other, so that
the impurity concentration distribution in the sample on the three
axes which are linearly independent of each other can be known. As
a result, the three-dimensional impurity concentration distribution
in the sample can be known.
[0050] Preferably, the magnetic resonance apparatus further has a
diamagnetic member provided near the magnetic field applying means
so as to increase the magnetic field intensity near the sample. In
this case, the diamagnetic member increases the magnetic field
intensity near the sample, so that the resolution can be further
improved.
[0051] Preferably, the sample is an impurity-doped semiconductor,
and the magnetic resonance apparatus measures the concentration of
an impurity in the semiconductor in accordance with the
electromagnetic wave detected by the detecting means.
[0052] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] FIG. 1 is a block diagram of a magnetic resonance apparatus
according to a first embodiment of the invention;
[0054] FIG. 2A is a cross section of the magnetic resonance
apparatus, FIG. 2B is a perspective view of coils and antennas, and
FIG. 2C is a plan view of the coil;
[0055] FIG. 3 is a schematic diagram showing Zeeman splitting;
[0056] FIG. 4 is a diagram for explaining the principle of MRI;
[0057] FIG. 5 is a perspective view showing the relations between a
sample and X, Y, and Z axes;
[0058] FIG. 6 is a schematic view showing the relations between a
sample and the X and Z axes;
[0059] FIGS. 7 to 9 are diagrams for explaining the intensity of a
magnetic field applied to a sample in a first process;
[0060] FIGS. 10 to 12 are diagrams for explaining the intensity of
a magnetic field applied to a sample in a second process;
[0061] FIG. 13 is a diagram showing a sample of which position is
changed by a driving mechanism;
[0062] FIGS. 14 to 16 are diagrams for explaining the intensity of
a magnetic field applied to a sample in a third process;
[0063] FIGS. 17 to 19 are diagrams for explaining the intensity of
a magnetic field applied to a sample in a fourth process;
[0064] FIG. 20 is a diagram showing a sample of which position is
changed by the driving mechanism;
[0065] FIGS. 21 to 23 are diagrams for explaining the intensity of
a magnetic field applied to a sample in a fifth process;
[0066] FIGS. 24 to 26 are diagrams for explaining the intensity of
a magnetic field applied to a sample in a sixth process;
[0067] FIG. 27 is a block diagram of a magnetic resonance apparatus
according to a second embodiment of the invention;
[0068] FIG. 28 is a perspective view of coils shown in FIG. 27;
[0069] FIGS. 29 to 31 are diagrams for explaining the intensity of
the magnetic field applied to the sample in the first process;
[0070] FIGS. 32 to 34 are diagrams for explaining the intensity of
the magnetic field applied to the sample in the second process;
[0071] FIG. 35 is a block diagram of a magnetic resonance apparatus
according to a third embodiment of the invention;
[0072] FIG. 36 is a cross section of a magnetic resonance apparatus
according to a fourth embodiment of the invention; and
[0073] FIG. 37 is a block diagram of a magnetic resonance apparatus
according to a fifth embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0074] Embodiments of the invention will now be described
hereinbelow with reference to the drawings.
First Embodiment
[0075] Referring to FIG. 1, a magnetic resonance apparatus 100
according to a first embodiment of the invention has: a cryostat 10
as holding means for cooling and holding a sample 1 including an
atomic nucleus having a magnetic moment; coils 21 and 22 as
magnetic field applying means for applying an inclined magnetic
field to sample 1 so that the intensity of the magnetic field has a
gradient to the reference axis of sample 1; a driving mechanism 30
as driving means for moving sample 1 to which the magnetic field is
applied with respect to the magnetic field; antennas 41 and 42 as
irradiating means for irradiating sample 1 to which the magnetic
field is applied with an electromagnetic wave; and antennas 41 and
42 as detecting means for detecting an electromagnetic wave emitted
from sample 1 irradiated with the electromagnetic wave.
[0076] Coils 21 and 22 Zeeman-splits the atomic nucleus by applying
the magnetic field to sample 1. The irradiating means and the
detecting means include common antennas 41 and 42. Magnetic
resonance apparatus 100 further includes a controller 50 as control
means for controlling the operation of driving mechanism 30.
[0077] Driving mechanism 30 can move sample 1 along three axes
linearly independent of each other. Sample 1 is an impurity-doped
semiconductor, and magnetic resonance apparatus 100 measures the
impurity concentration in the semiconductor in accordance with the
electromagnetic wave detected by antennas 41 and 42 as detecting
means.
[0078] Cryostat 10 is positioned in the center of the apparatus.
Cryostat 10 is fixedly placed on driving mechanism 30. Driving
mechanism 30 can move cryostat 10 and sample 1 held by cryostat 10
three-dimensionally.
[0079] A pair of antennas 41 and 42 are provided so as to sandwich
cryostat 10. Antennas 41 and 42 are provided so as to face each
other, irradiate sample 1 with an electromagnetic wave, and detect
the electromagnetic wave emitted from sample 1. As each of antennas
41 and 42, a loop antenna is used.
[0080] On both sides of antennas 41 and 42, a pair of coils 21 and
22 are provided. By passing a predetermined current into coils 21
and 22, a magnetic field is generated. Coils 21 and 22 apply the
magnetic field to sample 1.
[0081] Controller 50 is connected to cryostat 10, coils 21 and 22,
driving mechanism 30, and antennas 41 and 42 and sends signals to
those elements. A signal analyzer 60 is connected to cryostat 10,
driving mechanism 30, and antennas 41 and 42. Signal analyzer 60
and controller 50 are connected to each other.
[0082] Signal analyzer 60 is connected to a display 70. Signals
collected by signal analyzer 60 are processed in signal analyzer
60, and displayed as an image on display 70.
[0083] FIG. 2A is a cross section of a magnetic resonance
apparatus, FIG. 2B is a perspective view of coils and antennas, and
FIG. 2C is a plan view of the coil. Referring to FIGS. 2A to 2C,
the structure of magnetic resonance apparatus 100 according to the
invention will be described in detail.
[0084] (1) Coils 21 and 22
[0085] A magnet by which the strongest magnetic field can be
obtained at present is an air-cored superconducting magnet. Coils
21 and 22 construct an electromagnet. In proportional to the number
of turns of each of coils 21 and 22 and the current passed to coils
21 and 22, the stronger magnetic force can be generated.
[0086] As a wire material of coils 21 and 22 made of a
superconductor, Nb.sub.3Sn can be used. By passing a current of
about 250A to coils 21 and 22, a magnetic field having the
intensity of about 20T can be realized. In order to obtain a
stronger magnetic field, an oxide superconductor can be used as the
material of coils 21 and 22. Coils 21 and 22 are put in a thermal
insulation vessel filled with liquid helium. To suppress
evaporation of the liquid helium, the vessel is further put in a
thermal insulation vessel filled with liquid nitrogen.
[0087] In the case of a general MRI apparatus, the region of a
sample to be observed is large, and an uniform magnetic field has
to be applied to the whole region. Consequently, the main magnet
usually has to have a diameter of 10 cm or larger. In addition, an
electromagnet called "shim" for adjusting the uniformity is also
provided. In the invention, however, the region to which an ultra
high magnetic field is applied may be 1 cm.sup.2 or smaller, the
shim is not needed.
[0088] It is desirable to reduce the diameter D of each of coils 21
and 22 (refer to FIG. 2C) and the interval (t) (refer to FIG. 2B)
between coils 21 and 22 in order to increase the magnetic flux
density in the region of the sample 1. When the object to be
observed has high spatial density like a hydrogen atomic nucleus
contained in a photoresist applied on sample 1 which is a silicon
substrate, a coil having a diameter of about 30 cm may be used. In
the case of observing a small amount of atomic nuclei of arsenic or
the like implanted in the silicon substrate, it is advantageous to
use a coil having the diameter D of 1 cm or less as shown in FIG.
2C.
[0089] The intensity of the magnetic field becomes weaker with
distance from coils 21 and 22 for the reason that lines of magnetic
force expand in space and the density of the lines decreases. It is
therefore preferable that the distance between sample 1 and coils
21 and 22 is shorter. In order to avoid attenuation of the magnetic
field, it is advantageous that two magnetic poles (coils) of
different polarities are disposed so as to sandwich a region for
holding sample 1 and the interval between the magnetic poles is
narrowed as much as possible. In the case of using the silicon
substrate as sample 1, its thickness is 10 mm or less, and the
silicon substrate is preferably has a large size so that a
structure of a certain size can be mounted for controlling the
temperature of sample 1 or the like. It is preferable that the
interval t between coils 21 and 22 is 10 cm or less, and the
distance between sample 1 and each of the coils 21 and 22 is 5 cm
or less.
[0090] The electromagnet for applying an ultra high magnetic field
may perform a pulse operation. In this case, a magnetic field of
higher intensity as compared with a continuous operation can be
generated.
[0091] It is necessary to apply a gradient magnetic field to the
sample and scan the magnetic field intensity three-dimensionally.
For this purpose, there are two methods; a method of electrically
changing the magnetic field intensity by two or more sets of
electromagnets (coils), and a method of physically moving the
position of the sample while fixing the magnetic field. From a
viewpoint of the apparatus configuration, it is preferable to adopt
the former method, that is, the method of electrically changing the
magnetic field intensity by two or more sets of electromagnets and
obtain a spatial distribution of the nuclei to be observed by
signal analysis because the apparatus configuration becomes simple.
By narrowing the interval of the strong magnetic field areas, a
magnetic field gradient of 100 mT/m or more can be realized.
Preferably, the area (magnetic pole area) of the face of the coil
21 facing the coil 22 is set to 750 cm.sup.2 or less.
[0092] (2) Holding Means (Cryostat 10)
[0093] The lower the temperature of the sample is, the stronger an
MRI signal becomes from the principle of the signal. However, a
main object to be observed of a conventional MRI apparatus is a
hydrogen atomic nucleus contained in water components in a living
body, so that the temperature of the sample cannot be set to be
very low. In the invention, it is expected that the objects to be
observed are phosphorus, arsenic, or the like in the silicon
substrate, so that the holding means can be cooled by liquid
nitrogen, liquid helium, or the like. Consequently, a structure
capable of cooling the holding means by liquid nitrogen, liquid
helium, or the like can be used. A thermostat used for such a
purpose is usually cryostat 10.
[0094] Cryostat 10 has a three-layered structure and is constructed
by three walls 11, 12, and 13. The area surrounded by walls 11 and
12 is a thermal insulating layer, an interlayer between walls 12
and 13 is a coolant layer, and an area surrounded by wall 13 is a
sample chamber.
[0095] Sample 1 is loaded in the sample chamber. The sample chamber
is filled with liquid or gaseous helium 16. A heater 91 is
provided. Heater 91 may be provided on the outside of the sample
chamber. In the sample chamber, a helium introduction pipe 83 and a
helium exhaust pipe 84 are provided. Through helium introduction
pipe 83, liquid or gaseous helium 16 as a coolant is introduced
into the sample chamber. Through helium exhaust pipe 84, helium is
exhausted from the sample chamber to the outside.
[0096] Helium introduction pipe 83 and helium exhaust pipe 84 are
provided with valves 88 and 89, respectively. Valves 88 and 89 are
opened/closed by controller 50.
[0097] In the sample chamber, a temperature sensor 92 is provided
and connected to signal analyzer 60.
[0098] Wall 13 constructing the sample chamber is made of,
preferably, a material having high thermal conductivity. The
material has to have purity to a degree that sample 1 is not
contaminated since wall 13 comes into direct contact with the
backside of sample 1. When the temperature of sample 1 is increased
to be high while a contamination material is adhered, there is the
possibility that the contamination substance is diffused in sample
1. In the apparatus, the temperature is kept lower than the normal
room temperature.
[0099] The space between walls 12 and 13 is filled with liquid
nitrogen 15. The sample chamber is filled with helium 16. Liquid
nitrogen is introduced via a liquid nitrogen introduction pipe 81
and exhausted via a liquid nitrogen exhaust pipe 82. Liquid
nitrogen introduction pipe 81 and liquid nitrogen exhaust pipe 82
are provided with valves 86 and 87, respectively. The valves 86 and
87 are connected to controller 50. The space between walls 11 and
12 is under vacuum.
[0100] Since the temperature in the cryostat 10 becomes very low,
if there is moisture or oxygen in the cryostat 10, it is frozen or
liquefied. It is therefore preferable to provide an exhaust pump to
preliminarily eliminate the molecules. When the gas in the sample
chamber is replaced with helium, stability is obtained and
measurement can be performed easily. In the case of cooling sample
1 to the temperature of the liquid helium, the liquid helium can be
poured via helium introduction pipe 83 into the sample chamber.
Preferably, sample 1 is set to the temperature 100K or lower. The X
and Z axes of the face of sample 1 are parallel to the drawing
sheet and the Y axis is perpendicular to the drawing sheet.
[0101] (3) Irradiating Means and Detecting Means (Antennas 41 and
42)
[0102] Usually, a nuclear magnetic resonance phenomenon is excited
in MRI by a radio wave as the electromagnetic wave emitted from
loop antennas 41 and 42. The antennas 41 and 42 also serve as
detecting means for receiving resonance signals. The spatial
resolution of the MRI is inversely proportional to the frequency
width of the radio wave. The narrower the frequency distribution
is, the higher the resolution is. There is a maser as means for
obtaining an electromagnetic wave having a narrow frequency width.
Since the maser generates an electromagnetic wave having, as a
principle, a single frequency and high energy density, which can be
easily subjected to pulse operation, the maser is advantageous as
an exciting source of MRI.
[0103] A signal analyzer 60 similar to that in the conventional
another MRI apparatus can be used. Methods of measuring an MRI
signal include a saturation recovering method, an inversion
recovering method, and the like. In the invention, those methods
can be similarly used.
[0104] In MRI of which object to be observed is a living body,
since the object cannot be fixed, measurement of long observation
time cannot be performed. In the case where a not-living body is
measured, by adding up results of repetitive measurement, the SIN
ratio of the signal can be improved.
[0105] By the improvements as described above, much higher
sensitivity as compared with a conventional MRI apparatus is
obtained. In the case where 10.sup.16 atoms of phosphorus, boron,
or the like per 1 cm.sup.3 are distributed in a matrix of silicon,
a three-dimensional distribution can be observed at a spatial
resolution of about 0.1 .mu.m.
[0106] (4) Driving Mechanism
[0107] Driving mechanism 30 supports cryostat 10 and can move
cryostat 10 in linearly-independent three directions. Consequently,
the distribution of the impurity concentration in sample 1 can be
three-dimensionally measured.
[0108] The principle of the MRI will now be described. An atomic
nucleus having a spin quantum number I and a magnetic moment .mu.
mutually acts with a magnetic field B, and the degenerated energy
level is split to 2I+1. This is called Zeeman splitting. In the
case of a hydrogen atomic nucleus (having the number of proton of
1) as a representative object to be observed, the spin quantum
number I is {fraction (1/2+EE. In the magnetic field, the energy
level is split into two energy levels. )}
[0109] Referring to FIG. 3, in a state where there is no magnetic
field, hydrogen atomic nuclei 110 have a single energy level. In
contrast, in a state where there is a magnetic field, hydrogen
atomic nuclei 110 are split into two energy levels. The level on a
low energy side with the same orientation of a nuclear spin as that
of the magnetic field B is set as .alpha., and the level on a high
energy side with the opposite orientation is set as .beta.. The
ratio between the number N.alpha. of .alpha. spins and the number
N.beta. of .beta. spins is expressed as N.beta./N.alpha.=exp(-E/kT)
in accordance with the Boltzmann's distribution rule, where k
denotes a Boltzmann's constant, T indicates an absolute
temperature, and E expresses energy shown by 2 .mu.B.
[0110] When Zeeman-split atomic nucleus is irradiated with an
electromagnetic wave having a frequency v and the energy hv is
equal to E, a resonance phenomenon takes place. h denotes a
Planck's constant. By the resonance phenomenon, the electromagnetic
wave is emitted as a nuclear magnetic resonance (NMR) signal. The
intensity of the NMR signal is proportional to the number of
excitable atomic nuclei (N.alpha.-N.beta.)/2. The expression can be
modified as follows.
N.alpha.-N.beta.=N.alpha.(1-N.beta./N.alpha.)
=N.alpha.(1-exp(E/kT))
=N.alpha.(1-exp(-2 .mu.B/kT))
[0111] From the equation, it is understood that the higher the
magnetic field intensity is and the lower the temperature is, a
signal of a stronger intensity can be obtained.
[0112] N.alpha.-N.beta. of the protons (hydrogen atomic nuclei)
will be estimated. When flux density B is 2.35T (corresponding to
100 MHz) and T=300K, N.alpha.-N.beta.=0.00001. When there are
100,000 .beta. spins, .alpha. spins of 100,000+1 exist. Similarly,
when B=23.5T (corresponding to 1 GHz) and T=300K,
N.alpha.-N.beta.=0.00016. When B=2.35T (corresponding to 100 MHz)
and T=4.2K, N.alpha.-N.beta.=0.00114. It is understood that in the
above range of the magnetic field intensities and temperatures, the
number of excitable atomic nuclei, that is, the signal intensity is
almost proportional to the magnetic field intensity and inverse
proportional to the absolute temperature.
[0113] Even if a number of excitable atomic nuclei exist, if they
are not actually excited, no signal is obtained. In order to have a
signal of a high S/N ratio, therefore, an electromagnetic wave of
sufficient intensity has to be emitted.
[0114] To obtain the NMR signal by the method as described above,
the intensity of the magnetic field to be applied and the frequency
of the electromagnetic wave are set so that the resonance condition
is satisfied only in a very small space. By three-dimensionally
sweeping the very small space, a three-dimensional image of the
signal intensity can be obtained. This is the principle of the
MRI.
[0115] Usually, in the MRI, by applying a gradient magnetic field
to a sample and emitting an electromagnetic wave of a narrow band
frequency, the resonance condition is satisfied only in a
plane-shaped area. By analyzing signals obtained by sweeping the
plane area in the vertical and horizontal directions with respect
to the sample by a computer, a three-dimensional image is
reconstructed.
[0116] Referring to FIG. 4, for example, it is now assumed that the
intensity of the magnetic field having a gradient (a) in the X axis
direction is expressed by a straight line 121 and a resonance
condition h.nu..sub.0=2 .mu.B.sub.0 is satisfied at a position of a
magnetic field intensity B.sub.0 in the magnetic field. The atomic
nuclei contributed to emission of the NMR signal are distributed in
a plane perpendicular to the X axis. When the incident
electromagnetic wave does not have a single frequency but a
slightly varying frequency and the width of the frequency (upper
limit value-lower limit value) is d.nu., a width dX.sub.1 of the
position satisfying the resonance condition is expressed by the
following equation.
dX.sub.1=h.multidot.d.nu./2a.mu.
[0117] The narrower the width dv of the frequency is, the thinner
the distribution of the atomic nuclei contributed to the NMR signal
becomes, and the spatial resolution becomes higher. As shown by a
straight line 122, the sharper the gradient of the magnetic field
becomes, the thinner the distribution of the atomic nuclei
contributed to the NMR signal becomes, and the spatial resolution
becomes higher.
[0118] In order to improve the spatial resolution of the MRI,
therefore, there are the following methods.
[0119] (1) To make the gradient of the magnetic field applied to a
sample steeper.
[0120] (2) To narrow the frequency width of an electromagnetic wave
to be emitted.
[0121] As described above, by improving the spatial resolution of
the MRI, the number of atomic nuclei existing in the space where
the resonance condition is satisfied is decreased. It causes
reduction in intensity of the NMR signal. In order to realize the
MRI apparatus having high resolution, therefore, sufficient signal
intensity has to be assured. For this purpose, there are the
following methods.
[0122] (1) To increase the magnetic field intensity. Concretely, a
current passed to the coils constructing the electromagnet is
increased, the distance between magnetic poles is shortened, the
sample is positioned closer to the magnetic poles, and the
like.
[0123] (2) To set the temperature of the sample to be low.
Concretely, the sample chamber is covered with a thermal insulating
material and the inside is cooled by liquid nitrogen or liquid
helium.
[0124] (3) To increase the strength of an electromagnetic wave to
be emitted. Concretely, a maser is used as an electromagnetic wave
source. The maser oscillates at a single wavelength, so that it
also produces an effect of a narrow frequency band and an improved
spatial resolution.
[0125] A method of measuring a three-dimensional impurity
distribution will now be described.
[0126] Referring to FIGS. 5 to 9, in a first process, the magnetic
field intensity in the plane of Z=0 becomes B.sub.0, and a gradient
magnetic field is applied to a sample so as to satisfy the
resonance condition. The magnetic field is inclined to the Z axis
as a reference axis. On the plane expressed by Z=0, the resonance
condition is satisfied. When the electromagnetic wave is emitted
from antennas 41 and 42 in such a state, resonance occurs on the
plane of Z=0, and antennas 41 and 42 receive NMR signals. The
signal intensity added up on the plane of Z=0 can be consequently
calculated.
[0127] Referring to FIGS. 10 to 12, in a second process, driving
mechanism 30 slightly moves cryostat 10 and sample 1 from the state
shown in FIG. 2 toward the Z axis direction so that the resonance
condition is satisfied on the plane of Z=dZ. By the operation, the
resonance condition is satisfied on the plane of Z=dZ.
Consequently, resonance signals are obtained and the signal
intensity added up on the plane of Z=dZ is measured by antennas 41
and 42.
[0128] In FIG. 13, attached devices such as controller 50, signal
analyzer 60, and display are not shown. Referring to FIG. 13,
driving mechanism 30 moves sample 1. By the movement, the X and Z
axes in the plane of sample 1 become parallel to the drawing sheet,
and the Y axis becomes perpendicular to the drawing sheet. The X
axis extends in the direction from the coil 22 toward the coil
21.
[0129] Referring to FIGS. 14 to 16, in the third process, the
intensity of the magnetic field becomes B.sub.0 in the plane of
X=X.sub.0, the gradient magnetic field is applied to sample 1 so as
to satisfy the resonance condition. The magnetic field is inclined
to the X axis as a reference axis. The resonance condition is
satisfied on the plane expressed by X=X.sub.0. When the
electromagnetic wave is emitted from antennas 41 and 42 in such a
state, resonance takes place on the plane of X=X.sub.0, and
antennas 41 and 42 receive the NMR signals. Consequently, signal
intensity added up on the plane of X=X.sub.0 can be obtained.
[0130] Referring to FIGS. 17 to 19, in a fourth process, driving
mechanism 30 slightly moves cryostat 10 and sample 1 in the state
shown in FIG. 13 toward the X axis direction so that the resonance
condition is satisfied on the plane of X=X.sub.0+dX. By the
operation, the resonance condition on the plane of X=X.sub.0+dX is
satisfied. The NMR signal is obtained and signal intensity added up
on the plane of X=X.sub.0+dX is measured by antennas 41 and 42.
[0131] In FIG. 20, attached devices such as controller 50, signal
analyzer 60, and display are not shown. Referring to FIG. 20,
driving mechanism 30 moves sample 1. By the movement, the X and Y
axes in the plane of sample 1 become parallel to the drawing sheet,
and the Z axis becomes perpendicular to the drawing sheet. The Y
axis extends in the direction from coil 22 toward coil 21.
[0132] Referring to FIGS. 21 to 23, in a fifth process, the
intensity of the magnetic field becomes B.sub.0 in the plane of
Y=Y.sub.0, and the gradient magnetic field is applied to sample 1
so as to satisfy the resonance condition. The magnetic field is
inclined to the Y axis as a reference axis. The resonance condition
is satisfied on the plane expressed by Y=Y.sub.0. When the
electromagnetic wave is emitted from antennas 41 and 42 in such a
state, resonance occurs on the plane of Y=Y.sub.0, and antennas 41
and 42 receive the NMR signals. Consequently, signal intensity
added up on the plane of Y=Y.sub.0 can be obtained.
[0133] Referring to FIGS. 24 to 26, in a sixth process, driving
mechanism 30 slightly moves cryostat 10 and sample 1 in the state
shown in FIG. 20 toward the Y axis direction so that the resonance
condition is satisfied on the plane of Y=Y.sub.0+dY. By the
operation, the resonance condition on the plane of Y=Y.sub.0+dY is
satisfied. The NMR signal is obtained and signal intensity added up
on the plane of Y=Y.sub.0+dY is measured by antennas 41 and 42.
[0134] By moving the sample three-dimensionally in such a manner,
the three-dimensional distribution of the impurity in sample 1 can
be obtained without destroying sample 1. Further, since sample 1 is
cooled, the distribution of the impurity can be obtained with high
accuracy.
Second Embodiment
[0135] Referring to FIG. 27, a magnetic resonance apparatus 100
according to a second embodiment of the invention has: a cryostat
10 as holding means for cooling and holding sample 1 including an
atomic nucleus having a magnetic moment; coils 21, 22, 23, 24, 25,
and 26 as magnetic field applying means for sweeping a magnetic
field inclined so that the intensity of the magnetic field inclines
with respect to the reference axis of sample 1 and applying the
swept magnetic field to sample 1; antennas 41 and 42 as irradiating
means for irradiating sample 1 to which the magnetic field is
applied with an electromagnetic wave; and antennas 41 and 42 as
detecting means for detecting an electromagnetic wave emitted from
sample 1 irradiated with the electromagnetic wave. Coils 21 and 22
as magnetic field applying means Zeeman-splits the atomic nuclei by
applying the magnetic field to sample 1. The irradiating means and
the detecting means include the common antennas 41 and 42. Magnetic
resonance apparatus 100 further includes controller 50 as control
means for controlling the intensity of the magnetic field applied
by coils 21, 22, 23, 24, 25, and 26 as magnetic field applying
means to sample 1. Sample 1 is an impurity-doped semiconductor, and
magnetic resonance apparatus 100 measures the impurity
concentration in the semiconductor in accordance with the
electromagnetic wave detected by antennas 41 and 42.
[0136] The intensity of the magnetic field generated by each of
coils 21, 22, 23, 24, 25, and 26 can be changed by controller 50.
Specifically, controller 50 can change the intensity of the
magnetic field generated by each of coils 21 to 26 by adjusting the
current passed to each of coils 21 to 26. Consequently, each of
coils 21 to 26 can sweep the magnetic field and apply the swept
magnetic field to sample 1.
[0137] Referring to FIG. 28, coils are provided around cryostat 10.
The axis connecting coils 23 and 24 is the X axis, the axis
connecting coils 26 and 25 is the Y axis, and the axis connecting
coils 21 and 22 is the Z axis. The magnetic field applying means
includes coils 23 and 24 as a pair of first coils, coils 25 and 26
as a pair of second coils provided along the Y axis as the second
axis linearly independent of the X axis, and coils 21 and 22 as a
pair of third coils provided along the Z axis as a third axis
linearly independent of the X and Y axes. Although the X, Y, and Z
axes are orthogonal to each other in FIG. 28, the three axes do not
always have to be orthogonal to each other but may be linearly
independent of each other.
[0138] The not-shown antennas are provided between cryostat 10 and
coils 21 and 22.
[0139] Referring to FIGS. 29 to 31, in the first process, the
magnetic field intensity in the plane of X=X.sub.0 becomes B.sub.0,
and a gradient magnetic field is applied to the sample so as to
satisfy the resonance condition. The magnetic fields as shown in
FIGS. 29 to 31 can be realized by passing a current higher than
that passed to coil 23 to coil 21, passing the same current to
coils 25 and 26, and passing the same current to coils 21 and 22.
The magnetic field is inclined to the X axis as a reference axis.
On the plane expressed by X=X.sub.0, the resonance condition is
satisfied. When the electromagnetic wave is emitted from antennas
41 and 42 in such a state, resonance occurs on the plane of
X=X.sub.0, and antennas 41 and 42 receive NMR signals. The signal
intensity added up on the plane of X=X.sub.0 can be consequently
calculated.
[0140] Referring to FIGS. 32 to 34, in the second process, coils 23
and 24 sweep and apply the magnetic field to sample 1 so that the
resonance condition is satisfied on the plane of X=X.sub.0+dX.
Consequently, resonance signals are obtained and the signal
intensity added up on the plane of X=X.sub.0+dX is measured by
antennas 41 and 42.
[0141] By a similar method, the gradient magnetic field in the Y
axis direction and that in the Z axis direction are swept and
applied to sample 1. A distribution of the impurity in the plane
perpendicular to the Y axis and that in the plane perpendicular to
the Z axis can be obtained.
[0142] The magnetic resonance apparatus according to the second
embodiment of the invention constructed as described above produces
effects similar to those in the magnetic resonance apparatus
according to the first embodiment.
Third Embodiment
[0143] Referring to FIG. 35, a magnetic resonance apparatus 100
according to a third embodiment of the invention is different from
magnetic resonance apparatus 100 according to the first embodiment
with respect to the point that a maser 140 is used as irradiating
means. Maser 140 irradiates sample 1 in cryostat 10 with an
electromagnetic wave of a single wavelength. Maser 140 is connected
to controller 50. When a predetermined signal is received from
controller 50, maser 140 irradiates sample 1 with the
electromagnetic wave of the single wavelength.
[0144] Magnetic resonance apparatus 100 constructed in such a
manner produces effects similar to those of magnetic resonance
apparatus 100 according to the first embodiment. Further, since
maser 140 irradiates sample 1 with the electromagnetic wave of the
single wavelength, an effect of improved resolution of the impurity
concentration distribution is produced.
Fourth Embodiment
[0145] Referring to FIG. 36, a magnetic resonance apparatus 100
according to a fourth embodiment of the invention is different from
magnetic resonance apparatus 100 according to the first embodiment
with respect to the point that a diamagnetic member
(superconductor) 151 is provided near coils 21 and 22 so as to
increase the magnetic field intensity near sample 1.
[0146] The intensity of the magnetic field applied to sample 1 is
expressed by the density of lines of magnetic force passing through
sample 1. Consequently, when the lines of magnetic force passing on
the outside of sample 1 can be collected to sample 1, the magnetic
field intensity increases and the signal intensity can be
increased. For this purpose, the fact that the lines of magnetic
force cannot pass through a strong diamagnetic material can be
used. This is a phenomenon called a Meissner effect, and the
phenomenon often occurs in a superconductor. Although the
superconductors presently known produce the Meissner effect only at
an ultra low temperature, since the sample chamber in the invention
has the mechanism for holding an ultra low temperature, the
phenomenon can be used. Specifically, the superconductor is
disposed around the sample chamber and the ultra low temperature is
set, thereby lines of magnetic force which are supposed to pass
through the space in which the superconductor is placed can be
collected to sample 1. This is realized by using the phenomenon
that the magnetic permeability in the diamagnetic material becomes
zero. Any substance which is as long as a superconductor can be
used. As a substance displaying the Meissner effect at a relatively
high temperature, Y--Ba--Ca--O ceramics and bismuth ceramics are
known. Obviously, when a substance displaying strong diamagnetism
at room temperature is used, it is unnecessary to set an ultra low
temperature.
[0147] Magnetic resonance apparatus 100 according to the fourth
embodiment of the invention also produces effects similar to those
of magnetic field resonance apparatus 100 according to the first
embodiment. Further, since the lines of magnetic force can be
collected to sample 1, the resolution can be further improved.
Fifth Embodiment
[0148] Referring to FIG. 37, a magnetic resonance apparatus 100
according to the fifth embodiment of the invention is different
from magnetic resonance apparatus 100 according to the second
embodiment with respect to the point that diamagnetic members 151
and 152 are provided near coils 21 and 22 and maser 140 is provided
as irradiating means.
[0149] Magnetic resonance apparatus 100 according to the fourth
embodiment of the invention also produces effects similar to those
of magnetic field resonance apparatus 100 according to the fourth
embodiment. Further, since maser 140 irradiates the sample 1 with
the electromagnetic wave of the single wavelength, an effect such
that the resolution of the impurity concentration distribution is
further improved is produced.
[0150] According to the invention, the distribution of atomic
nuclei in a sample can be obtained at high resolution without
destroying the sample.
[0151] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *